More recently, non-contact inspection methods in nondestructive evaluation (NDE) have been receiving substantial attention compared with contact or liquid-coupled inspection methods. In particular, it is more practical and efficient to employ a non-contact inspection method if the article under inspection is wood, paper product, porous material, or hot metallic material. Unlike the contact inspection methods, the non-contact inspection methods use only gas or air as a coupling medium, so that there are no risks of contamination of the test article. In addition, the unique characteristics of air or gas as a coupling medium, such as the low sound wavespeed and minimal fluid loading, have encouraged the development of more non-contact inspection applications. Air-coupled ultrasound inspection applications have and continue to develop in various areas, including materials inspection[1-2], characterization[3-5], and ultrasonic imaging[6-7].
Most non-contact inspection methods employ either conventional piezoelectric (PZT) transducers or capacitive micromachined transducers. When a PZT transducer is used as a primary probe in air, it encounters very large acoustic impedance mismatch at the boundary between the piezoelectric element and the surrounding air or gas boundary. Because of this, impedance matching must be employed to improve the acoustic energy transmission in gaseous environments. Attempts to remedy this problem have been limited to success in narrow bandwidth operations. In addition, the application of a matching layer limits the overall bandwidth of the device.
Capacitive ultrasonic transducers consist of a thin metallized polymer membrane and conducting backplate. Compared to the piezoelectric transducers, the capacitive ultrasonic transducers have much smaller acoustic impedance mismatch between the membrane and air, owing to the very small mechanical impedance of a thin membrane. This arrangement makes a capacitive ultrasonic transducer ideal for coupling into air. The vibration of the membrane generates ultrasound in air. Receiving the vibrating sound signals is achieved using the same transducer as a reciprocal device.
Recently, microfabrication techniques have been used to fabricate capacitive air-coupled ultrasonic transducers[8-10]. Indeed, these techniques provide a means to fabricate the capacitive air-coupled transducers with low fabrication cost, high reliability, relatively high sensitivity, and reasonably wide bandwidth. Details of their operation and performance are reported elsewhere[8, 11-12].
With this high popularity and interest, additional effort is being invested in the development of transducer focusing for capacitive air-coupled transducers. A focused transducer can provide much higher transducer sensitivity than a non-focusing planar device. So far, this goal has largely eluded investigators, except for the use of mirrors[5], cylindrical focusing[13], and a Fresnel zone plate[14]. One group has attempted slightly to deform Si-wafers[15]. Mirrors provide only limited bandwidth and leave one dimension unfocused. Still, the Fresnel zone plate approach has inherent narrowband frequency response, image degradation by the generation of side lobes, and no specific design guidelines to decide radii of zone plates for ultrasound. Moreover, the cylindrical focusing technique relies heavily on surface conditions. Si-wafers have proven difficult to handle owing to the fragility and brittleness of the silicon. They also leave one dimension unfocused and suffer from bandwidth limitations. Because brittle silicon wafers have customarily been used to fabricate a focused ultrasonic transducer capacitor, little progress has been made in the development of a new backplate material. Therefore, the challenge still remains to fabricate a focused capacitive ultrasonic transducer.